[Feature]: iOS setting to open PR links in the GitHub app instead of in-app

[RishabhTayal]

Summary

Add a setting in the iOS app that lets me choose where pull request links open: in the T3 Code iOS app (current behavior) or in the GitHub app.

Motivation

I review and merge PRs in the GitHub app on my phone. Tapping a PR link in T3 Code currently opens it inside T3 Code, so I lose the GitHub app's native review, comment, and merge flows and have to copy the link and switch apps by hand.

Proposed behavior

• New toggle/picker in iOS Settings, e.g. Open PR links in: T3 Code (default) / GitHub app.
• When set to GitHub app, tapping a PR link (for example from a thread's PR badge or a link in a message) hands the github.com/<owner>/<repo>/pull/<n> URL to the system so the GitHub app's universal link handling opens it.
• If the GitHub app isn't installed, fall back to the in-app view or Safari.
• Default stays as today, so nothing changes for existing users.

Alternatives considered

Long-pressing a link and choosing "Open in GitHub" works, but it is slow and easy to forget.